1. Field of the Invention
The present invention relates to the field of machine building for a mining industry and, more particularly, to the bits for hole drilling with an air or gas cleaning of a hole face. The improvement of this design relates to the support unit which serves for rotatably mounting each cone on a leg trunnion and fixing it against axial displacement.
2. Description of the Prior Art and its Disadvantages
Development of the cone drill bits equipped with hard-alloy inserts has made the rock cone drilling the prevailing method for hard minerals exploration by open-cut mining and for the construction industry replacing a churn drilling and, partially, air percussion, percussion-rotary, auger drilling etc.
A specific feature of the rock cone bit drilling is cleaning a hole face with compressed air.
When in operation, a drill bit is subjected to high specific loads with increased vibration caused by high strength and fracturing of the drilled rock. Moreover, heat removal from the cone support units as compared to a water or solution flush drilling is imbarrased.
That's why, a commonly known cone bit support unit (hereinafter a support) including a radial roller bearing adjacent to the trunnion base, locking radial thrust ball bearing in the center and radial thrust roller bearing or radial rolling bearing adjacent to the end of the trunnion has become widely used.
Russia and Ukraine plants produce rock drill bits under a "roller-ball-roller" (RBR) scheme, i.e. on the rolling bearings. Companies of U.S., Sweden and other countries produce rock drill bits under a "roller-ball-thrust and radial plain bearing" (RBP) scheme. In latter case the main load portion is transmitted through roller and ball bearings. End plain bearings are of an auxiliary importance.
The most advanced developments of "Baker Hughes" and "Security Dresser" U.S. companies (1,2) are the rock drill bits with a RBRP support scheme. In these designs the end radial plain bearing is replaced with a rolling bearing, from the plain bearings remaining only thrust bearings in a form of a journal at the trunnion end and an intermediate ring thrust bearing arranged on a shoulder of the locking ball bearing directed to a bit centerline.
It is commonly known that for cooling the support and removing the drilled rock cuttings from it, there is used blowing the support with compressed air. This measure, nevertheless, is insufficient for reliable heat removal. That's why, it is desirable to reject completely from using the plain bearings in the support unit.
A sliding friction coefficient is almost by an order of magnitude higher than a rolling friction coefficient, particularly, in the non-sealed support working without lubrication. Use of the plain bearings in the support in order to avoid overheating compels to reduce a power applied to the bit and transmitted to the hole face mainly through limiting a revolution rate to the level of 40-60 rpm in cases where it is impossible to reduce a load on the bit due to the drilled rock strength. Altogether, it results in reduction of a mechanical rate of penetration.
Apart from the thermal overstrain, the plain bearing use leads to an increase of the reactive moment, i.e. an anti-torque moment with respect to the cone revolution. And this results in developing an additional transverse component at the cone inserts which in its turn causes their failure and wear in a form of blunted areas since drilled rocks are characterised by high abrasiveness and strength.
Analysis of bits weared in rock drilling showed that the weakest unit in existing designs of supports under the RBR scheme is a locking radial thrust ball bearing. It is natural, since in this support scheme the ball bearing is a sole bearing receiving the total load component which is directed along the trunnion centerline whereas a component directed perpendicular to the trunnion centerline is received by all the support bearings including the ball bearing.
Inadequate efficiency of the ball bearing manifests itself in three aspects. Firstly, on its working surfaces on the trunnion and balls there appear fatique contact damages earlier than in roller bearings. Secondly, due to the overstrain the balls crack which results in a subsequent disastrous support wear. And, in the third place, a trunnion working shoulder for the ball bearing (directed to the hole wall), as a rule, at a definite working stage is either sheared off due to fatigue strains or it bends aside when there appears a creep restraining the most powerful radial bearing arranged adjacent to the trunnion base. In a case of the shoulder failure, the peripheral bearing rollers loose a direction and turn around in the race and, in a case of a roller restraining, this bearing becomes a peculiar plain bearing. Both cases result in a forced support destruction and the bit failure.
In Russia, numerous attempts have been made to eliminate overloading the locking ball bearing by reducing the radial roller bearings number in the support and simultaneous increase of the radial thrust ball bearings number up to two or even three units. During a certain period of time there were produced rock bits of size 214 mm with a double-row ball support, i.e. the ball bearings have been arranged adjacent to the trunnion base and in the center, with radial roller bearing being disposed at the trunnion end (a BBR support scheme). However, said support has not been successful since in this case the ball bearing located adjacent to the trunnion base became overloaded. In this case its overloading originated from a radial load directed perpendicular to the trunnion centerline. As a result, the previous RBR support scheme was accepted again and it is used up to present moment.
The European and U.S. companies have developed a combined rolling/plain support in which a load component directed along the trunnion centerline is received by a radial thrust ball bearing interacting with two or one thrust plain bearing(s) and a part of the radial load is accepted by the end radial plain bearing in addition to the main peripheral roller bearing.
The thrust plain bearings use in the support essentially eliminates failures of the working shoulder of a ball bearing or its bending aside giving preference to said support over the above-mentioned RBR support. The end radial plain bearing leaves more free space in contrast to the roller bearing and makes it easier to solve the problem of arranging the equipment elements for efficient rock disintegration in the face central part.
However, when drilling with air cleaning of the hole face, this support, as it was mentioned above, requires a moderate operation in order to eliminate the plain bearings overheating.